The present invention relates to a photopolymerizable recording material comprising a base material, a photopolymerizable layer and a cover layer which is relatively impermeable to atmospheric oxygen. The material is suitable in particular for producing printing plates, in particular planographic printing plates.
Printing plates of the type mentioned are known for example from U.S. Pat. No. 3,458,311. The cover layers described therein consist of water-soluble polymers, for example polyvinyl alcohol, polyvinylpyrrolidone, gelatin or gum arabic. 0f these substances, polyvinyl alcohol is preferred, since its barrier effect in respect of oxygen is very high. However, it has been found that stored photopolymerizable printing plates having a cover layer made of polyvinyl alcohol lose some of their light sensitivity in the course of weeks and months. It has also been observed that the protective effect strongly decreases with increasing temperature and increasing atmospheric humidity. It has indeed been possible to show that the oxygen permeability of polyvinyl alcohol strongly increases with temperature and atmospheric humidity. This disadvantage is particularly serious in the case of highly light-sensitive photopolymerizable mixtures containing photooxidizable substances and special synergistic initiator combinations. Such mixtures are described in earlier German Patent Applications P 37 10 279.6, P 37 10 281.8, P 37 10 282.6 and P 37 43 454.3.
It is also known to add to the photopolymerizable layer itself substances, for example tin salts or antioxidants, in order to trap the oxygen diffusing into the layer. Such additions are described, for example, in a paper by E. T. Denisov et al. in Chem. Rev. 1987, pp. 1313-1357. Since such compounds usually have undesirable side effects and in particular reduce the light sensitivity, they have not become established in practice.